Paper laminates comprising tungsten treated titanium dioxide having improved photostability

ABSTRACT

This disclosure relates to a resin-impregnated, opaque, cellulose pulp-based sheet comprising an inorganic particle, wherein the inorganic particle comprises at least about 0.002% of tungsten, based on the total weight of the inorganic particle, and has a photostability ratio (PSR) of at least about 2, as measured by the Ag +  photoreduction rate, and color as depicted by an L* of at least about 97.0, and b* of less than about 4. The disclosure also relates to paper laminates prepared from these resin-impregnated, opaque, cellulose pulp-based sheets.

BACKGROUND OF THE DISCLOSURE

1. Field of the Disclosure

The present disclosure relates to resin-impregnated, opaque, cellulosepulp-based sheet and paper laminates, and more particularly toresin-impregnated, opaque, cellulose pulp-based sheet and paperlaminates prepared therefrom comprising tungsten.

2. Background of the Disclosure

Paper laminates are in general well-known in the art, being suitable fora variety of uses including table and desk tops, countertops, wallpanels, floor surfacing, tableware and the like. Paper laminates havesuch a wide variety of uses because they can be made to be extremelydurable, and can be also made to resemble (both in appearance andtexture) a wide variety of construction materials, including wood,stone, marble and tile, and can be decorated to carry images and colors.

Typically, the paper laminates are made from papers by impregnating thepapers with resins of various kinds, assembling several layers of one ormore types of laminate papers, and consolidating the assembly into aunitary core structure while converting the resin to a cured state. Thetype of resin and laminate paper used, and composition of the finalassembly, are generally dictated by the end use of the laminate.

Decorative paper laminates can be made by utilizing a printed decorativepaper layer as upper paper layer and various support paper layers in theunitary core structure. The decorative paper is typically highly opaqueso that the appearance of the support layers below the decorative paperdoes not adversely impact the appearance of the decorative paperlaminate. A decorative paper is also known as a décor paper.

To achieve required abrasion, scuff, and mar resistance, typically, aseparate overlay is used as the top layer for paper laminates. Anoverlay usually comprises the same resin as the one that is used for theresin impregnated decorative paper.

A paper laminate has been made by applying to the outer layer of acomposite structure a mixture of an additive amount of a fluorourethaneadditive, available from E. I. du Pont de Nemours and Company and amelamine resin slurry. Paper laminates may be produced by both low- andhigh-pressure lamination processes.

Various methods can be employed to provide paper laminates bylow-pressure lamination. For example, a single opening, quick cyclepress can be used where one or more resin-saturated paper sheets arelaminated to a sheet of plywood typically with a 1A face, particleboard, or fiberboard.

In a high-pressure lamination process, a melamine overlay and a melamineresin-impregnated décor paper are usually laminated onto a phenolicsheet, which provides additional mechanical support. For example, a“continuous laminator” can be used where one or more layers of theresin-saturated paper are pressed into a unitary structure as the layersmove through continuous laminating equipment between plates, rollers orbelts. One or two laminated sheets (continuous web or cut to size) maybe pressed onto a particle or fiberboard, etc. and a “glue line” used tobond the laminated sheet to the board. Single or multiple openingpresses may also be employed which contain several laminates.

The decor paper in such paper laminates generally comprises aresin-impregnated, cellulose pulp-based sheet, with the pulp being basedpredominantly on hardwoods such as eucalyptus, sometimes in combinationwith minor amounts of softwood pulps. Pigments (such as titaniumdioxide) and fillers are added in amounts generally up to and includingabout 45 wt. % (based on the total dry weight prior to resinimpregnation) to obtain the required opacity. Other additives such aswet-strength, retention, sizing (internal and surface) and fixing agentsmay also be added as required to achieve the desired end properties ofthe paper. The resin can be a thermosetting resin selected from thegroup consisting of a polymer of diallyl phthalate, epoxide, ureaformaldehyde, urea-acrylic acid ester. copolyester, melamineformaldehyde, melamine phenol formaldehyde, dicyandiamide-formaldehyde,urethane, curable acrylic, unsaturated polyester and phenol formaldehydeand mixtures thereof.

Titanium dioxide pigments are prepared using either the chloride processor the sulfate process. In the preparation of titanium dioxide pigmentsby the vapor phase chloride process, titanium tetrachloride. TiCl₄, isreacted with an oxygen containing gas at temperatures ranging from about900° C. to about 1600° C., the resulting hot gaseous suspension of TiO₂particles and free chlorine is discharged from the reactor and must bequickly cooled below about 600° C., for example, by passing it through aconduit, i.e., flue, where growth of the titanium dioxide pigmentparticles and agglomeration of said particles takes place.

It is known to add various substances, such as silicon compounds andaluminum compounds, to the reactants in order to improve the pigmentaryproperties of the final product. Aluminum trichloride added during theprocess has been found to increase rutile in the final product, andsilicon tetrachloride that becomes silica in the final product has beenfound to improve carbon black undertone (CBU), particle size and pigmentabrasion. It is useful to be able to add elements to the titaniumdioxide particles. However, the process and materials to be added toimprove properties of the titanium dioxide particles may be hazardous.

One method of adding elements to the surface of a particle is byimpregnation with a solution containing the element. This is difficultto do with pyrogenically prepared metal oxide particles since theproperties of the pyrogenically produced metal oxides change uponcontact with a liquid medium.

A need exists for a low cost approach for preparing paper laminatescomprising pyrogenically prepared metal oxide particles, particularlytitanium dioxide particles, comprising elements such as tungsten thatprovide improved photostability without changing the color of theproduct.

SUMMARY OF THE DISCLOSURE

In a first aspect, the disclosure provides a resin-impregnated, opaque,cellulose pulp-based sheet comprising inorganic particles, typicallyinorganic metal oxide or mixed metal oxide particles, more typicallytitanium dioxide (TiO₂) particles, comprising at least about 0.002% oftungsten, more typically at least about 0.004% of tungsten, and stillmore typically at least about 0.01% of tungsten, and most typically atleast about 0.05% of tungsten, based on the total weight of theinorganic particles, wherein the inorganic particles, have aphotostability ratio (PSR) of at least about 2, more typically at leastabout 4, and still more typically at least 10, as measured by the Ag⁺photoreduction rate, and color as depicted by an L* of at least about97.0, more typically at least about 98, and most typically at leastabout 99.0, and b* of less than about 4, and more typically less thanabout 3. Typically the inorganic particles, more typically inorganicmetal oxide or mixed metal oxide particles, and most typically titaniumdioxide particles, comprising tungsten may further comprise alumina inthe amount of about 0.06 to about 5% of alumina, more typically about0.2% to about 4% of alumina, still more typically about 0.5% to about 3%of alumina, and most typically about 0.8% to about 2%, based on thetotal weight of the inorganic particles.

In a second aspect, the disclosure provides a paper laminate comprisinga resin-impregnated, opaque, cellulose pulp-based sheet, wherein theresin-impregnated, opaque, cellulose pulp-based sheet comprisesinorganic particles, typically inorganic metal oxide or mixed metaloxide particles, more typically titanium dioxide (TiO₂) particles,comprising at least about 0.002% of tungsten, more typically at leastabout 0.004% of tungsten, and still more typically at least about 0.01%of tungsten, and most typically at least about 0.05% of tungsten, basedon the total weight of the inorganic particles, wherein the inorganicparticles, have a photostability ratio (PSR) of at least about 2, moretypically at least about 4, and still more typically at least 10, asmeasured by the Ag⁺ photoreduction rate, and color as depicted by an L*of at least about 97.0, more typically at least about 98, and mosttypically at least about 99.0, and b* of less than about 4, and moretypically less than about 3. Typically the inorganic particles, moretypically inorganic metal oxide or mixed metal oxide particles, and mosttypically titanium dioxide particles, comprising tungsten may furthercomprise alumina in the amount of about 0.06 to about 5% of alumina,more typically about 0.2% to about 4% of alumina, still more typicallyabout 0.5% to about 3% of alumina, and most typically about 0.8% toabout 2%, based on the total weight of the inorganic particles. Thepaper laminate further comprises a dried overlay.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic illustration showing the process for preparingtitanium dioxide (TiO₂).

DETAILED DESCRIPTION OF THE DISCLOSURE

This disclosure relates to a paper laminate comprising aresin-impregnated, opaque, cellulose pulp-based sheet, wherein theresin-impregnated, opaque, cellulose pulp-based sheet comprisesinorganic particles, typically inorganic metal oxide or mixed metaloxide particles, more typically titanium dioxide (TiO₂) particles,comprising at least about 0.002% of tungsten, more typically at leastabout 0.004% of tungsten, and still more typically at least about 0.01%of tungsten, and most typically at least about 0.05% of tungsten, basedon the total weight of the inorganic particles. These inorganicparticles have a photostability ratio (PSR) of at least about 2, moretypically at least about 4, and still more typically at least 10, asmeasured by the Ag⁺ photoreduction rate, and color as depicted by an L*of at least about 97.0, more typically at least about 98, and mosttypically at least about 99.0, and b* of less than about 4, and moretypically less than about 3. Typically the inorganic particles, moretypically inorganic metal oxide or mixed metal oxide particles, and mosttypically titanium dioxide particles, comprising tungsten may furthercomprise alumina in the amount of about 0.06 to about 5% of alumina,more typically about 0.2% to about 4% of alumina, still more typicallyabout 0.5% to about 3% of alumina, and most typically about 0.8% toabout 2%, based on the total weight of the inorganic particles, and thepaper laminate made therefrom.

The paper laminate typically comprises a dried overlay and a base sheetwherein at least one of the dried overlay and the base sheet cancomprise a resin-impregnated, opaque, cellulose pulp-based sheet. Thebase sheet can comprise a phenolic core or engineered wood comprisingsubstrate such as particle or fiber board. The dried overlay and thebase sheet can be laminated together utilizing a low pressure or a highpressure lamination process. The paper laminate may further comprisecomponents to make it abrasion resistant.

Resin-Impregnated, Opaque, Cellulose Pulp-Based Sheet:

The resin-impregnated, opaque, cellulose pulp-based sheet is also knownin the industry as Décor paper. The cellulose pulp used in thepulp-based sheet comprises pulp predominantly from hardwoods such aseucalyptus, sometimes in combination with minor amounts of softwoodpulps. Pigments (such as titanium dioxide, more typically rutiletitanium dioxide comprising tungsten and in addition alumina) andfillers can be added in amounts generally up to and including about 60wt. %, more typically about 20% to about 40%, (based on the total dryweight prior to resin impregnation) to obtain the required opacity.Other additives such as wet-strength, retention, sizing (internal andsurface) and fixing agents may also be added as required to achieve thedesired end properties of the Décor paper. Resins used to impregnate thepapers are typically thermosetting resins. Examples of suitablethermosetting resins include, without limit, polymers of diallylphthalate, epoxide, urea formaldehyde, urea-acrylic acid ester.copolyester, melamine formaldehyde, melamine phenol formaldehyde,dicyandiamide-formaldehyde, urethane, unsaturated polyester, curableacrylic and phenol formaldehyde and mixtures thereof. In somesituations, the resin used to impregnate this decorative sheet maycontain abrasive inorganic particles selected from the group consistingof aluminum oxide or silicon oxide and mixtures thereof.

This resin impregnated, opaque, cellulose pulp-based sheet may contain aprint, pattern design or solid color and these are generated using knowntechniques. Some such techniques include various well-known analog anddigital printing methods to impart desired coloration and designs asrequired for the particular end use. Analog printing methods such asscreen printing are particularly suitable for large runs and repetitivepatterns. Digital printing methods such as inkjet printing areparticularly suitable for short runs and customized patterning.

Some suitable resin-impregnated, opaque, cellulose pulp-based sheets areavailable from Mead Westvaco (11013 West Broad Street, Glen Allen, Va.23060), as, solid colored Duoply® papers or printbase Primebase® papers.

Dried Overlay

The dried overlay can be wear resistant and the dried overlay can beused in both low pressure and high pressure lamination processes toprovide improved resistance to abrasive wear. The dried overlay can beof varying thickness and can be low opacity, more typicallysubstantially optically transparent.

The dried overlay can comprise a thermosetting resin or can be aresin-impregnated, opaque, cellulose pulp-based sheet as describedabove. The thermosetting resin used in the dried overlay can besubjected to a pre-cure step prior to the lamination process which alsoincludes a curing step. The term “pre-cure” is used to mean that thecure of the resin particles has been advanced either to the maximumdegree possible or at least to a stage of cure where the melt viscosityof the cured resin particles is sufficiently high to prevent theseparticles from melting and flowing under usual laminating conditions andthus undesirably saturating into the décor paper or otherresin-impregnated, opaque, cellulose pulp-based sheet, during thelamination step to form the paper laminate.

The resins are typically thermosetting resins. Examples of suitablethermosetting resins include, without limit polymer of diallylphthalate, epoxide, urea formaldehyde, urea-acrylic acid ester.copolyester, melamine formaldehyde, melamine phenol formaldehyde,dicyandiamide-formaldehyde, urethane, curable acrylic, unsaturatedpolyester and phenol formaldehyde and mixtures thereof. More typicallythe resin used in the dried overlay is a formaldehyde-melamine polymer.

Especially when the dried overlay is not a resin impregnated, opaque,cellulose pulp-based sheet, the resin used to impregnate theresin-impregnated opaque cellulose pulp-based sheet typically has thesame or substantially the same index of refraction as the resin in thedried overlay. More typically, the resin used in the dried overlay isthe same resin used to impregnate the resin-impregnated opaque cellulosepulp-based sheet.

The dried overlay further comprises a binding material, selected from agroup consisting of microcrystalline cellulose, carboxyl methylcellulose, sodium alginate and mixtures thereof.

Optionally, the dried overlay further comprises mineral particles,usually ranging is size from about 20 to about 35 μm, comprisingaluminum oxide, silicon oxide, or the mixture thereof, to furtherimprove abrasion resistance.

The dried overlay can be transparent after curing.

The dried overlay can be made by processes well known in the papermaking industry, by forming a suspension of the resin and the bindingmaterial together and drying the suspension to form the dried overlay.Optionally additional ingredients can be employed such as the mineralparticles and opacifier, if the dried overlay is to be opaque.

The dried overlay can also be made by applying a thick layer ofpre-cured thermosetting resin particles to the decorative sheet, asdisclosed in U.S. Pat. No. 5,545,476.

Some suitable dried overlays, specifically the melamine-containingoverlays are commercially available form Wilsonart International ofFletcher North Carolina.

Other Components of the Paper laminate

The paper laminate can comprise other components such as a phenolic coresheet, engineered wood sheet, such as particle board or fiber board orplywood. The phenolic core sheet typically comprises a plurality ofphenolic resin-impregnated Kraft papers which are laminated together.Glues can also be included usually as seam sealants, for example, a hotwax-oil emulsion. Other suitable glues are made of acrylic polymer,polyvinylacetate, and polychloroprene and commercially available fromWilsonart International of Fletcher N.C.

Treated Particle:

It is contemplated that any inorganic particle, and in particularinorganic particles that are photoactive, will benefit from thetreatment of this disclosure. By inorganic particle it is meant aninorganic particulate material that becomes dispersed throughout a finalproduct such as a polymer melt or coating or paper laminate compositionand imparts color and opacity to it. Some examples of inorganicparticles include but are not limited to ZnO, ZnS, BaSO₄, CaCO₃, TiO₂,Lithopane, white lead. SrTiO₃, etc.

In particular, titanium dioxide is an especially useful particle in theprocesses and products of this disclosure. Titanium dioxide (TiO₂)particles useful in the present disclosure may be in the rutile oranatase crystalline form. They are commonly made by either a chlorideprocess or a sulfate process. In the chloride process. TiCl₄ is oxidizedto TiO₂ particles. In the sulfate process, sulfuric acid and orecontaining titanium are dissolved, and the resulting solution goesthrough a series of steps to yield TiO₂. Both the sulfate and chlorideprocesses are described in greater detail in “The Pigment Handbook”,Vol. 1, 2nd Ed., John Wiley & Sons, NY (1988), the teachings of whichare incorporated herein by reference. The particle may be a pigment ornanoparticle.

By “pigment” it is meant that the titanium dioxide particles have anaverage size of less than 1 micron. Typically, the particles have anaverage size of from about 0.020 to about 0.95 microns, more typically,about 0.050 to about 0.75 microns and most typically about 0.075 toabout 0.50 microns. By “nanoparticle” it is meant that the primarytitanium dioxide particles typically have an average particle sizediameter of less than about 100 nanometers (nm) as determined by dynamiclight scattering that measures the particle size distribution ofparticles in liquid suspension. The particles are typically agglomeratesthat may range from about 3 nm to about 6000 nm.

The titanium dioxide particle can be substantially pure titanium dioxideor can contain other metal oxides, such as alumina. Other metal oxidesmay become incorporated into the particles, for example, byco-oxidizing, post-oxidizing, co-precipitating titanium compounds withother metal compounds or precipitating other metal compounds on to thesurface of titanium dioxide particles. These are typically hydrous metaloxides. If co-oxidized, post-oxidized, precipitated or co-precipitatedthe amount of the metal oxide is about 0.06 to about 5%, more typicallyabout 0.2% to about 4%, still more typically about 0.5% to about 3%, andmost typically about 0.8% to about 2%, based on the total weight of thetitanium dioxide particles. Tungsten may also be introduced into theparticle using co-oxidizing, or post-oxidizing. If co-oxidized orpost-oxidized at least about 0.002 wt. % of the tungsten, moretypically, at least about 0.004 wt. %, still more typically at leastabout 0.01 wt. % tungsten, and most typically at least about 0.05 wt. %may be present, based on the total particle weight.

Process for Preparing Treated Titanium Dioxide Particles

The process for producing titanium dioxide particle comprises:

-   -   a) mixing of chlorides of, titanium, tungsten or mixtures        thereof; wherein at least one of the chlorides is in the vapor        phase;    -   (b) oxidizing the chlorides of, titanium, tungsten or mixtures        thereof; and    -   (c) forming titanium dioxide (TiO₂) particles comprising at        least about 0.002% of tungsten, more typically at least about        0.004% of tungsten and still more typically at least about 0.01%        of tungsten, and most typically at least about 0.05% of        tungsten, based on the total weight of the titanium dioxide        particles. These titanium dioxide particles have a        photostability ratio (PSR) of at least 2, more typically at        least 4, and still more typically at least 10, as measured by        the Ag⁺ photoreduction rate, and color as depicted by an L* of        at least about 97.0, more typically at least about 98, and most        typically at least about 99.0, and b* of less than about 4, and        more typically less than about 3. Typically the titanium dioxide        particles comprising tungsten further comprise alumina in the        amount of about 0.06 to about 5% of alumina, more typically        about 0.2% to about 4% of alumina, still more typically about        0.5% to about 3% of alumina, and most typically about 0.8% to        about 2%, based on the total weight of the titanium dioxide        particles.

Methods known to one skilled in the art may be used to add tungsten tothe titanium dioxide particles. In one specific embodiment, tungsten maybe added to the titanium dioxide particle from an alloy comprisingtungsten. As shown in FIG. 1, the alloy 11 and chlorine 12 are added tothe generator 10. This reaction can occur in fluidized beds, spoutingbeds, packed beds, or plug flow reactors. The inert generator bed maycomprise materials such as silica sand, glass beads, ceramic beads, TiO₂particles, or other inert mineral sands. The alloy comprising aluminum,titanium or mixtures thereof and tungsten, 11, reacts in the generator10 according to the following equations:

2Al+3Cl₂→2AlCl₃+heat

Ti+2Cl₂→TiCl₄+heat

W+3Cl₂→WCl₆+heat

Al₁₂W+21Cl₂→12AlCl₃+WCl₆+heat

The heat of reaction from the chlorination of the aluminum or titaniummetal helps provide sufficient heat to drive the kinetics of thereaction between chlorine and one or more of the other elements.

Titanium tetrachloride 17 may be present during this reaction to absorbthe heat of reaction. The chlorides formed in-situ comprise chlorides ofthe tungsten and chlorides of aluminum such as aluminum trichloride,chlorides of titanium such as titanium tetrachloride or mixturesthereof. The temperature of the reaction of chlorine with the alloyshould be below the melting point of the alloy but sufficiently highenough for the rate of reaction with chlorine to provide the requiredamount of chlorides to be mixed with the TiCl₄.

Typical amounts of chlorine used in step (a) are about 0.4% to about20%, more typically about 2% to about 5%, by weight, based on the totalamount of all reactants. Typical amounts of titanium tetrachloride areabout 75% to about 99.5% added in step (a) and (b), and more typicallyabout 93% to about 98%, by weight, based on the total amount of allreactants.

The reaction of chlorine with the alloy occurs at temperature of above190° C., more typically at temperature of about 250° C. to about 650°C., and most typically at temperatures of about 300° C. to about 500° C.In one specific embodiment where the metal is Ti the reaction occurs attemperature of above 50° C. (bp of TiCl₄=136° C.), more typically attemperature of about 200° C. to about 1000° C., and most typically attemperatures of about 300° C. to about 500° C.

The chlorides formed in the in-situ step 13 flows into an oxidationreactor 14 and titanium tetrachloride 15 is then added to the chlorides,such that titanium tetrachloride is present in a major amount. Vaporphase oxidation of the chlorides from step (a) and titaniumtetrachloride is by a process similar to that disclosed, for example, inU.S. Pat. Nos. 2,488,439, 2,488,440, 2,559,638, 2,833,627, 3,208,866,3,505,091, and 7,476,378. The reaction may occur in the presence ofneucleating salts such as potassium chloride, rubidium chloride, orcesium chloride.

Such reaction usually takes place in a pipe or conduit, wherein oxygen16, titanium tetrachloride 15 and the in-situ formed chloridescomprising chlorides of tungsten and chlorides of aluminum such asaluminum trichloride, chlorides of titanium such as titaniumtetrachloride or mixtures thereof 13 are introduced at a suitabletemperature and pressure for production of the treated titanium dioxide.In such a reaction, a flame is generally produced.

Downstream from the flame, the treated titanium dioxide produced is fedthrough an additional length of conduit wherein cooling takes place. Forthe purposes herein, such conduit will be referred to as the flue. Theflue should be as long as necessary to accomplish the desired cooling.Typically, the flue is water cooled and can be about 50 feet (15.24 m)to about 3000 feet (914.4 m), typically about 100 feet (30.48 m) toabout 1500 feet (457.2 m), and most typically about 200 feet (60.96 m)to 1200 feet (365.76 m) long.

The following Examples illustrate the present disclosure. All parts,percentages and proportions are by weight unless otherwise indicated.

EXAMPLES

Photostability ratio (PSR) is the rate of photoreduction of Ag+ by TiO₂particles without tungsten (control samples) divided by the rate ofphotoreduction of Ag+ by the otherwise same TiO₂ particles comprisingtungsten. The rate of photoreduction of Ag+ can be determined by variousmethods. A convenient method was to suspend the TiO₂ particles in 0.1 MAgNO₃ aqueous solution at a fixed ratio of TiO₂ to solution, typically1:1 by weight. The suspended particles were exposed to UV light at about0.2 mW./cm² intensity. The reflectance of visible light by thesuspension of TiO₂ particles was monitored versus time. The reflectancedecreased from the initial value to smaller values as silver metal wasformed by the photoreduction reaction, Ag⁺−>Ag^(o). The rate ofreflectance decrease versus time was measured from the initialreflectance (100% visible reflectance with no UV light exposure) to areflectance of 90% after UV exposure; that rate was defined as the rateof Ag⁺ photoreduction.

Color as measured on the CIE 1976 color scale, L*, a*, and b*, wasmeasured on pressed pellets of dry TiO₂ powder.

Comparative Example 1

Titanium dioxide made by the chloride process comprising 1.23% aluminaby weight and having an L*a*b* color index of (99.98, 0.60, 2.13) and arate of Ag⁺ photoreduction of 0.0528 sec⁻¹ was fired under flowingoxygen at 4° C./min to 1000° C. and held at temperature for 3 hours;furnace cooled to 750° C. and held at temperature for 1 hour; furnacecooled to 500° C. and held at temperature for 3 hours; furnace cooled to250° C. and held at temperature for 3 hours; and finally furnace cooledto room temperature. After firing the sample had an L*a*b* color indexof (99.15, −0.45, 2.17) and a rate of Ag⁺ photoreduction of 0.1993sec⁻¹.

Comparative Example 2

Titanium dioxide made by the chloride process comprising 0.06% aluminaby weight and having an L*a*b* color index of (99.43, −0.58, 1.36) and aphotoractivity rate of 0.3322 was fired under flowing oxygen at 4°C./min to 1000° C. and held at temperature for 3 hours; furnace cooledto 750° C. and held at temperature for 1 hour; furnace cooled to 500° C.and held at temperature for 3 hours; furnace cooled to 250° C. and heldat temperature for 3 hours; and finally furnace cooled to roomtemperature. After firing the sample had an L*a*b* color index of(97.71, −0.03, 1.89) and a photoactivity rate of 0.2229 sec⁻¹.

Example 3

Titanium dioxide similar to that described in Comparative Example 1 waswell mixed with various amounts of ammonium tungstate,(NH₄)₁₀W₁₂O₄₁·5H₂O, to give samples having the W contents listed below.These samples were fired as described in Comparative Example 1. Afterfiring the samples had L*a*b* color and photostability ratios (PSR) asgiven in the following table:

W (wt. %) L* a* b* PSR 0.0 99.15 −0.45 2.17 1.0 0.34 99.00 −0.71 2.723.0 1.72 98.56 −0.82 3.17 10.4 3.44 98.41 −0.90 3.11 211.4

The increased incorporation of W clearly enhanced photostability up toroughly a factor of 200 while the color was only minimally affected.

Example 4

Titanium dioxide similar to that described in Comparative Example 1 wasimpregnated via incipient wetness with various amounts of ammoniumtungstate, (NH₄)₁₀W₁₂O₄₁·5H₂O, to give samples having the W contentslisted below. These samples were fired as described in ComparativeExample 1. After firing the samples had L*a*b* color and photostabilityratios as given in the following table:

W (wt. %) L* a* b* PSR 0.0 98.16 0.02 2.09 1.0 0.34 97.97 −0.02 2.53 2.21.72 97.52 −0.15 2.79 10.0 3.44 97.41 −0.53 3.34 67.4

The increased incorporation of W clearly enhanced photostability up toroughly a factor of 67 while the color index was only minimallyaffected.

Example 5

Titanium dioxide similar to that described in Comparative Example 2 waswell mixed with amounts of ammonium tungstate, (NH₄)₁₀W₁₂O₄₁·5H₂O, togive samples having the W contents listed below. These samples werefired as described in Comparative Example 1. After firing the sampleshad L*a*b* color and photostability ratios as given in the followingtable:

W (wt. %) W L* a* b* PSR 0.0 0.0 97.71 −0.03 1.89 1.0 0.34 1x 97.73−0.21 2.19 4.3 1.72 5x 97.18 −0.56 1.94 139.0 3.44 10x  97.03 −0.83 2.45113.8

The increased incorporation of W clearly enhanced photostability up toroughly a factor of 140 while the color index was only minimallyaffected.

Comparative Example 6

Titanium dioxide similar to that described in Comparative Example 1 waswell mixed with various amounts of ammonium molybdate,(NH₄)₆Mo₇O₂₄·4H₂O, to give samples having the Mo contents listed below.These samples were fired as described in Comparative Example 1. Afterfiring the samples had L*a*b* color and photostability ratios as givenin the following table:

Mo (wt. %) L* a* b* PSR 0.0 98.76 −0.37 2.48 1 0.18 94.08 −3.45 17.96314.8 0.91 93.77 −4.47 30.45 no rate 1.83 91.89 −5.27 35.82 no rate

The increased incorporation of Mo clearly enhanced photostability to thepoint where, at the higher Mo concentrations, the photostability ratiocould not be determined. However, the material took on a decidedlyyellow coloration clearly compromising its use as a white pigment.

Comparative Example 7

Titanium dioxide similar to that described in Comparative Example 1 wasimpregnated via incipient wetness with various amounts of ammoniummolybdate, (NH₄)₆Mo₇O₂₄·4H₂O, to give samples having Mo to Al atomicratios of 0.1, 0.5, and 1.0 versus 0.0 for the undoped control. Thesesamples were fired as described in Comparative Example 1. After firingthe samples had L*a*b* color and photostability ratios as given in thefollowing table:

Mo (wt. %) L* a* b* PSR 0.0 97.79 −0.19 2.57 1.0 0.18 92.62 −3.61 24.15862.3 0.91 92.66 −4.21 31.63 1188.0 1.83 90.74 −4.92 37.94 no rateThe incorporation of Mo clearly enhanced photostability to the pointwhere, at the highest Mo concentration, the photostability ratio couldnot be determined. However, the material took on a decidedly yellowcoloration clearly compromising its use as a white pigment.

Example 8

Whatman #1 filter paper is impregnated with a slurry consisting of 10wt. % of the dry titanium dioxide samples having W contents as listed inExample 3 and a 50 wt. % aqueous solution of Kauramin® 773 impregnatingresin (melamine formaldehyde powder). The impregnated paper is dried ina convection oven at 230° F. Laminate lay-ups are constructed betweentwo steel caul plates. From the bottom up, the construction is asfollows:

-   -   a) single overlay sheet (LK2050FK MELAMINE IMPREGNATED 20# BASE        WEIGHT, WHITE OVERLAY)    -   b) single white backing sheet (L2028050 MELAMINE IMPREGNATED 80#        BASE WEIGHT, WHITE BACKER)    -   c) three sheets of Kraft paper (C99N5OCG PHENOLIC IMPREGNATED        99# BASE WEIGHT, KRAFT PAPER)    -   d) single white backing sheet (see (b) above)    -   e) two sheets of the Whatman #1 filter paper impregnated with        TiO₂ slurry    -   f) single overlay sheet (see (a) above)

The laminate is formed using a Carver press heated to 300° F. under aforce of 36,000 pounds for six minutes.

1. A resin-impregnated, opaque, cellulose pulp-based sheet comprising aninorganic particle, wherein the inorganic particle comprises at leastabout 0.002% of tungsten, based on the total weight of the inorganicparticle, and has a photostability ratio (PSR) of at least about 2, asmeasured by the Ag⁺ photoreduction rate, and color as depicted by an L*of at least about 97.0, and b* of less than about
 4. 2. Theresin-impregnated, opaque, cellulose pulp-based sheet of claim 1 whereinthe inorganic particle is an inorganic metal oxide or mixed metal oxideparticle.
 3. The resin-impregnated, opaque, cellulose pulp-based sheetof claim 2 wherein the inorganic metal oxide particle is titaniumdioxide.
 4. The resin-impregnated, opaque, cellulose pulp-based sheet ofclaim 3 further comprising a resin.
 5. The resin-impregnated, opaque,cellulose pulp-based sheet of claim 4 wherein the resin is athermosetting resin.
 6. The resin-impregnated, opaque, cellulosepulp-based sheet of claim 5 wherein the thermosetting resin is a polymerof diallyl phthalate, epoxide, urea formaldehyde, urea-acrylic acidester. copolyester, melamine formaldehyde, melamine phenol formaldehyde,dicyandiamide-formaldehyde, urethane, unsaturated polyester, curableacrylic and phenol formaldehyde or mixtures thereof.
 7. Theresin-impregnated, opaque, cellulose pulp-based sheet of claim 3 whereinthe amount of the titanium dioxide in the resin-impregnated, opaque,cellulose pulp-based sheet is up to and including about 60 wt. %, basedon the total dry weight of the cellulose pulp-based sheet prior to resinimpregnation.
 8. The resin-impregnated, opaque, cellulose pulp-basedsheet of claim 7 wherein the amount of the titanium dioxide in the theresin-impregnated, opaque, cellulose pulp-based sheet ranges from about20 to about 40 wt. %, based on the total dry weight of the cellulosepulp-based sheet prior to resin impregnation.
 9. The resin-impregnated,opaque, cellulose pulp-based sheet of claim 3 wherein tungsten ispresent in the amount of at least about 0.004%, based on the totalweight of the inorganic particle.
 10. The resin-impregnated, opaque,cellulose pulp-based sheet of claim 3 wherein the photostability ratio(PSR) is at least about
 4. 11. The resin-impregnated, opaque, cellulosepulp-based sheet of claim 3 wherein L* is at least about
 98. 12.resin-impregnated, opaque, cellulose pulp-based sheet of claim 3 whereinb* is less than about
 3. 13. The resin-impregnated, opaque, cellulosepulp-based sheet of claim 3 wherein tungsten is added to the titaniumdioxide particle by cooxidation or post-oxidation.
 14. Theresin-impregnated, opaque, cellulose pulp-based sheet of claim 3 whereintungsten is added to the titanium dioxide particle from an alloycomprising tungsten.
 15. The resin-impregnated, opaque, cellulosepulp-based sheet of claim 3 wherein the titanium dioxide particlefurther comprises alumina in the amount of about 0.06 to about 5% basedon the total weight of the titanium dioxide particle.
 16. A paperlaminate prepared from a resin-impregnated, opaque, cellulose pulp-basedsheet comprising an inorganic particle, wherein the inorganic particlecomprises at least about 0.002% of tungsten, based on the total weightof the inorganic particle, and has a photostability ratio (PSR) of atleast about 2, as measured by the Ag⁺ photoreduction rate, and color asdepicted by an L* of at least about 97.0, and b* of less than about 4.17. The paper laminate of claim 16 further comprising a dried overlay.18. The paper laminate of claim 16 wherein the inorganic particle in thecoating composition is an inorganic metal oxide or mixed metal oxideparticle.
 19. The paper laminate of claim 18 wherein the inorganic metaloxide particle is titanium dioxide.
 20. The paper laminate of claim 16wherein tungsten is present in the amount of at least about 0.004%,based on the total weight of the inorganic particle.
 21. The paperlaminate of claim 19 wherein the titanium dioxide particle furthercomprises alumina in the amount of about 0.06 to about 5% based on thetotal weight of the titanium dioxide particle.